This invention relates to improvements in acoustic well logging methods and apparatus and more particularly to improvements in techniques for transmitting and receiving acousitc energy in a borehole.
In acousitc well logging apparatus presently in use, a longitudinally extending support member carrying a plurality of acoustic transmitters and receivers is supported by a cable in the borehole for movement therethrough to investigate the surrounding earth formations. In the past, the support member has been centered in the borehole with transducers spaced apart along its length. In this case, the transducers have been of the cylindrical type with their axes aligned along the longitudinally extending axis of the support member which carries them. In one type of measuring system, a transducer emits acoustic energy into the borehole for passage through the formation to a pair of nearby acoustic reeceiver transducers to enable a measurement of the acoustic travel time between the two receivers. In another type of system, the amplitude difference or ratio between selected half cycles of the signals respectively produced by the two receivers is measured. In still another type of system, the waveform of the signal produced by one receiver is displayed or recorded for subsequent analysis or processing.
While the centered acoustic well logging devices have generally provided good results, they nevertheless suffer from a number of inherent disadvantages. For example, they are very sensitive to the positioning of the support member in the borehole. Thus, a tilted support member would create a situation where the travel time to one receiver transducer would be different from that to the other receiver thus creating a measurement error. This problem can be alleviated to a great extent by the use of compensation techniques of the type shown in U.S. Pat. Nos. 3,257,639 granted to F. P. Kokesh on June 21, 1966, or 3,207,256 granted to R. B. Blizard on Sept. 21, 1965.
In addition to tilting of the support member, it can be offcentered in the borehole thus causing in the reception signal a waveform distortion and an amplitude diminution because of elementary acoustic rays arriving out of phase at different circumferential points of each individual receiver transducer. It is possible, under certain formation conditions, that the resulting signal so produced by the receiver will be distorted and attenuated to the point that accurate determination of the time of arrival of acoustic energy at such receiver is not possible. This phase distortion caused by offcentering of the support member in the borehole can be compensated for by mounting at least the receivers in a pad designed to be applied against the borehole wall and in some cases shielding the back or non-borehole wall engaging side of the pad from acoustic energy. A pad mounted acoustic logging device can be found in U.S. Pat. No. 3,406,776 granted to L. Henry on Oct. 22, 1968 or U.S. Pat. No. 3,542,150 granted to A. H. Youmans et al. on Nov. 24, 1970.
Even when the acoustic transducers are mounted on a borehole wall engaging pad member, it has been found that the phase distortion of the emitted or received elementary acoustic rays can produce an appreciable error in the resulting measurement. As a matter of fact, as the transducers have a significant relative dimension versus the acoustic wave length normally used, the form of the signal produced at a receiver will be dependent on the phase difference of elementary waves, and therefore on factors other than the formation parameter which is to be measured. For example, if acousitc travel time is to be measured, such phase distortion of elementary acoustic waves may cause the amplitude of the half cycle usually selected for measurement to be less than the detection threshold, thus altering the measured travel time. (In making acoustic travel time measurements, a detection threshold is used to prevent the erroneous detection of noise.)
It is therefore an object of the present invention to provide new and improved methods and apparatus for transmitting and/or receiving acoustic energy in a borehole.
It is another object of the present invention to provide new acoustic well logging methods and apparatus wherein one or more acoustic transducers are constructed and arranged in an acoustic well logging apparatus in a manner which minimizes waveform distortion and amplitude diminution of the received signal.
In accordance with the apparatus features of the present invention, an acoustic transducer means is located in a longitudinally extending support member or in a pad member adapted for movement through a borehole. When in the borehole, the support member supports the transducer means whether it is in the pad or support member. The transducer means is supported near a portion of a borehole wall and is arranged such that the surface of the transducer means defines a curved line along a plane which extends longitudinally and radially with respect to the longitudinal axis of the support or pad member (that axis which is supposed to be parallel to the longitudinal axis of the borehole) to produce a substantially semi-circular radiation pattern over a selected portion of the region surrounding the transducer means.
In accordance with the method features of the present invention, acoustic energy is emitted at a first location in a borehole and a portion of the emitted energy received at a second location spaced from the first location to produce an electrical signal representative of the received energy. The emitted energy has a substantially semi-circular radiation pattern over a selected region surrounding the first location so that the produced electrical signal will be relatively unaffected by the particular value of the limit refraction angle for acoustic energy entering or leaving the borehole.
For a better understanding of the present invention, together with other and further objects thereof, reference is had to the following description taken in connection with the accompanying drawings, the scope of the invention being pointed out in the appended claims.